One example of a coupling of a trailer to a highway tractor which provides a vertical axis about which the trailer can swing relative to the tractor comprises a fifth wheel supported on a chassis frame of a tractor and a kingpin on a trailer which locks to the fifth wheel. As the trailer swings about the vertical axis, the angle between its longitudinal centerline and that of the tractor changes. That angle is referred to as the articulation angle.
Aerodynamic drag is imposed on a tractor-trailer by a gap between a cab of the tractor and a body of the trailer as the tractor-trailer travels along a roadway. The dimensions of the gap and the traveling speed of the tractor-trailer influence the magnitude of aerodynamic drag. In general, the greater the size of the gap and the greater the traveling speed, the greater the aerodynamic drag.
Steering the tractor of a moving tractor-trailer into a turn initially causes the path of a trailer rear axle group to depart from the paths of travel of front and rear axle groups of the tractor because the tractor will turn faster into the turn than the trailer. That departure results in the articulation angle increasing as turning radius of a rear axle group of the trailer becomes greater than the turning radii of the front and rear axle groups of the tractor. If the steering angle of the tractor is held constant as a turn continues, the trailer's longitudinal centerline swings back toward the tractor's longitudinal centerline and the turning radius of the trailer rear axle group will approach the turning radii of the front and rear axle groups of the tractor.
While minimizing distance between a rear of a tractor cab and a front of a trailer body is often an appropriate practice for minimizing gap-induced aerodynamic drag, the gap size should be sufficiently generous to avoid the trailer body hitting the tractor cab and any other structures in the vicinity of the gap over a potential range for the articulation angle.
A fifth wheel may be fastened to a tractor chassis frame so that its fore-aft position relative to the tractor cab is non-adjustable, or it may be mounted via an adjustment mechanism which allows its fore-aft position to be set to any of multiple distances from the tractor cab which inherently set the size of the gap between the tractor and the trailer. Whether the distance from a trailer body to a tractor cab is fixed or can be set to any one of several different distances, the distance influences aerodynamic drag on the moving tractor-trailer.
Various known methods and devices are available for reducing aerodynamic drag caused by a gap between a highway tractor and a trailer. Setting position of a fifth wheel is one example, but that procedure is typically done prior to, and not during, travel of a tractor-trailer on a roadway.
Another example is the deployment of structures which are sometimes referred to as fairings. Fairings may be mounted on a tractor and/or a trailer and may be deployed between tractor and trailer to reduce aerodynamic drag without changing gap size while the tractor-trailer is traveling.